Documentation: fix selftests related file refs

Make refs to selftests files valid including:
  - watchdog-test.c
  - dnotify_test.c
Signed-off-by: NTom Saeger <tom.saeger@oracle.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/dnotify.txt

@@ -62,7 +62,7 @@ disabled, fcntl(fd, F_NOTIFY, ...) will return -EINVAL.
 
 Example
 -------
-See Documentation/filesystems/dnotify_test.c for an example.
+See tools/testing/selftests/filesystems/dnotify_test.c for an example.
 
 NOTE
 ----

Documentation/watchdog/hpwdt.txt

@@ -15,7 +15,7 @@ Last reviewed: 05/20/2016
 
  Watchdog functionality is enabled like any other common watchdog driver. That
  is, an application needs to be started that kicks off the watchdog timer. A
- basic application exists in the Documentation/watchdog/src directory called
+ basic application exists in tools/testing/selftests/watchdog/ named
  watchdog-test.c. Simply compile the C file and kick it off. If the system
  gets into a bad state and hangs, the HPE ProLiant iLO timer register will
  not be updated in a timely fashion and a hardware system reset (also known as

Documentation/watchdog/pcwd-watchdog.txt

@@ -25,7 +25,7 @@ Last reviewed: 10/05/2007
 
  If you want to write a program to be compatible with the PC Watchdog
  driver, simply use of modify the watchdog test program:
- Documentation/watchdog/src/watchdog-test.c
+ tools/testing/selftests/watchdog/watchdog-test.c
 
 
  Other IOCTL functions include: